There are no published protocols for the transformation and regeneration of Allium species. The Allium crop species are probably the most economically important vegetable species for which transformation technology is unavailable. For other major vegetable crops, confirmed transformation systems have been produced.
Initially, many monocotyledons were thought to be unsusceptible to Agrobacterium-mediated transformation. The development of direct gene transfer techniques soon led to bombardment being the favoured method of monocotyledon transformation. However, direct gene transfer is not without its problems. Often, low transformation frequencies and a high frequency of unusual integration patterns has been observed in transgenic plants. Recently, Agrobacterium-mediated transformation of monocotyledons has gained favour and many monocotyledonous species (including rice; wheat, barley, maize and sugarcane) have now been transformed using this method. A key component in the success of these systems has been the transfer of DNA to callus cell types (usually derived from the pre culture of embryo tissue) followed by regeneration from these callus cells using precise post transformation selection protocols. Transformation of Allium callus is not useful as regeneration from callus is extremely difficult.
Recently, Haseloff (1997) has modified the gfp gene to enhance its use as a transgenic marker gene in viable plant systems. Green fluorescent protein (GFP) enables researchers to follow precisely the fate of any cells expressing this gene and so optimise post transformation cell survival. Such a system has been useful in the development of the onion transformation protocol reported here.
As monocotyledons, the Allium species were predisposed to be recalcitrant to transformation. Onions (Allium cepa L) are a crop with diverse environmental requirements. It has, therefore, been relatively understudied with respect to the application of biotechnology. There are only a few reports of DNA delivery to Alliums (Klein 1987; Dommisse et al. 1990; Eady et al. 1996; Barandiaran et al. 1998). Three workers used direct gene transfer whilst Dommisse et al. (1990) demonstrated that Agrobacterium-mediated transformation may be possible. Recently some reports of regeneration protocols for Alliums that are appropriate for transformation study have become available (Hong and Deberg 1995; Xue et al. 1997; Eady et al. 1998; Saker 1998). Only one report exists on the development of potential selective agents for use in Allium transformation (Eady and Lister 1998a).